Thermostat malfunction detecting system for engine cooling system

ABSTRACT

Malfunction of an engine coolant thermostat is detected from the engine side coolant temperature by the subsequent behavior of the coolant temperature. When an open-malfunction occurs, the coolant temperature becomes considerably different from that normally expected for times at which the thermostat is expected to be closed. When a closure-malfunction occurs, the coolant temperature becomes considerably different from that normally expected for times at which the thermostat is expected to be opened. Alternatively, the malfunction may be detected from the difference between the engine side coolant temperature and the radiator side coolant temperature.

CROSS REFERENCE TO RELATED APPLICATIONS

This application relates to and incorporate herein by reference JapanesePatent Applications No. 8-336579 filed on Dec. 17, 1996, No. 8-344749filed on Dec. 25, 1996 and No. 9-283208 filed on Oct. 16, 1997.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a thermostat malfunction detectingsystem of an engine cooling system for detecting whether a thermostatfor controlling the temperature of coolant of an engine is inmalfunction or not.

2. Related Art

Generally, a thermostat which opens/closes in correspondence to thetemperature of a coolant (cooling water) is provided in a coolantcirculating path for circulating the coolant between a water jacketwithin an engine and a radiator in the water-cooled type engine. It isclosed from the start of the engine until when the warm-up operation iscompleted to halt the circulation of the coolant to raise thetemperature of the coolant quickly to the required temperature range andto improve the fuel consumption and to reduce noxious exhaust noxiousexhaust noxious exhaust emission. The thermostat automatically openswhen the temperature of the coolant on the engine side exceeds therequired temperature range to circulate the low temperature coolant onthe radiator side to the engine side to lower or maintain thetemperature of the coolant on the engine side to the requiredtemperature range.

As modes of malfunction of the thermostat, there are an open-malfunctionduring which the thermostat is kept opened and a closure-malfunctionduring which it is kept closed. When the open-malfunction occurs, thecold coolant within the radiator is circulated to the engine from thebeginning of start even during the cold start time during which theengine is started while it is cold, so that the temperature of thecoolant on the engine side is hampered from rising after the start, thusretarding the warm-up of the engine and increasing the fuel consumptionand noxious exhaust noxious exhaust noxious exhaust emission. When theclosure-malfunction occurs, the cold coolant on the radiator side is notcirculated even when the temperature of the coolant on the engine sideexceeds the required temperature range, so that there is a possibilitythat the temperature of the coolant on the engine side keeps rising,causing an over-heat of the engine in the end.

Thus, there has been a possibility that even when the thermostat has theopen-malfunction, a driver continues to drive a vehicle without knowingit for a long period of time and continues to drive the vehicle untilengine overheats when it has the closure-malfunction.

It is noted that there has been a technology of providing coolanttemperature sensors at the inlet and outlet of the radiator,respectively, to evaluate the heat radiating performance of the radiatorbased on the temperature of coolant at the inlet and outlet of theradiator to detect the deterioration of the radiator as disclosed inJapanese Patent Application Laid-Open No. Hei. 4-19329. However, becausethe thermostat opens/closes automatically in correspondence to thetemperature of the coolant on the engine side, the malfunction of thethermostat cannot be detected even if the coolant temperature on theradiator side which is not related to the to opening/closing operationof the thermostat is detected at the two spots as disclosed. Still more,the cost becomes high because two temperature sensors have to beprovided anew on the radiator side.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide athermostat malfunction detecting system of a cooling system of aninternal combustion engine which detects the malfunction of thethermostat accurately at relatively low cost.

According to a first aspect of the present invention, a thermostatmalfunction detecting system of detects, based on the behavior of acoolant temperature on the engine side, the coolant temperature on thepath for circulating the coolant on the engine side from the thermostat(engine side coolant temperature) and diagnoses the thermostat whetherit has an open-malfunction by which it is not closed and is kept opened(open-malfunction) based on the engine side coolant temperature detectedin a temperature range in which the thermostat is normally closed.Because the behavior of the engine side coolant temperature is largelydifferent during the normal time and during the open-malfunction in thetemperature range in which the thermostat is normally closed, thethermostat may be diagnosed whether it has the open-malfunctionaccurately from the behavior of the engine side coolant temperature inthis temperature range. Still more, because the coolant temperature maybe detected by using the coolant temperature sensor for controlling theengine provided in the conventional engine, no new coolant temperaturesensor needs to be added to the engine control system.

When a closure-malfunction by which the thermostat is not opened and iskept closed occurs, the thermostat is not opened, the coolant is notcirculated and the engine side coolant temperature continues to rise up.Accordingly, the thermostat malfunction detecting system diagnoses thethermostat whether it has the closure-malfunction based on the detectedengine side coolant temperature in the temperature range in which thethermostat is normally opened. Because the behavior of the engine sidecoolant temperature is largely different during the normal time andduring the closure-malfunction in the temperature range in which thethermostat is normally opened, the thermostat may be diagnosedaccurately whether it has the closure-malfunction from the behavior ofthe engine side coolant temperature in this temperature range.

According to a second aspect of the present invention, a thermostatmalfunction detecting system detects the coolant temperature on the pathfor circulating the coolant on the engine side from the thermostat(engine side coolant temperature) as well as a coolant temperature onthe path for circulating the coolant on the radiator side from thethermostat (radiator side coolant temperature) and diagnoses thethermostat whether it has a malfunction based on the engine side coolanttemperature and the radiator side coolant temperature. Thereby, themalfunction of the thermostat can be detected accurately. Still more,because the engine side coolant temperature may be detected by using thecoolant temperature sensor for controlling the engine which has beenprovided in the conventional engine and just a radiator side coolanttemperature detecting means needs to be added anew to the engine controlsystem, the structure can be relatively simple and the increase of thecost is minimized.

According to a third aspect of the present invention, a thermostatmalfunction detecting system determines that the thermostat has amalfunction when a coolant temperature drops below a malfunctiondiscriminating temperature which is lower than the thermostat closingtemperature after when the coolant temperature reaches a warm-upcompletion temperature. That is, when the drop of the coolanttemperature does not stop even if the coolant temperature drops belowthe thermostat closing temperature, it may be considered that theopen-malfunction has occurred. Thereby, the open-malfunction of thethermostat may be detected by using the conventional coolant temperaturesensor provided on the coolant circulating path of the engine and no newsensor or the like needs to be added, satisfying the demand on thereduction of the cost.

BRIEF DESCRIPTION OF DRAWINGS

Other objects, features and advantages of the present invention willbecome more apparent in the following description and the accompanyingdrawings in which like numerals refer to like parts. In the accompanyingdrawings:

FIG. 1 is a block diagram showing the structure of the whole enginecooling system according to a first embodiment of the present invention;

FIG. 2 is a time chart showing a method for diagnosing anopen-malfunction (1);

FIG. 3 is a flow chart showing a flow of processing steps of anopen-malfunction diagnosing program for diagnosing the open-malfunction(1);

FIG. 4 is a time chart showing a method for diagnosing anopen-malfunction (2);

FIG. 5 is a flow chart showing a flow of processing steps of anopen-malfunction diagnosing program for diagnosing the open-malfunction(2);

FIG. 6 is a flow chart showing a flow of processing steps of anopen-malfunction diagnosing program for diagnosing an open-malfunction(3);

FIG. 7 is a time chart showing a method for diagnosing anopen-malfunction (4);

FIG. 8 is a flow chart showing a flow of processing steps of anopen-malfunction diagnosing program for diagnosing the open-malfunction(4);

FIG. 9 is a time chart showing a method for diagnosing anopen-malfunction (5);

FIG. 10 is a flow chart showing a flow of processing steps of a firsthalf part of an open-malfunction diagnosing program for diagnosing theopen-malfunction (5);

FIG. 11 is a flow chart showing a flow of processing steps of a secondhalf part of the open-malfunction diagnosing program for diagnosing theopen-malfunction (5);

FIG. 12 is a time chart showing a method for diagnosing aclosure-malfunction (1);

FIG. 13 is a flow chart showing a flow of processing steps of aclosure-malfunction diagnosing program for diagnosing theclosure-malfunction (1);

FIG. 14 is a flow chart showing a flow of processing steps of a programfor integrating quantity of heat generated by an engine;

FIG. 15 is a table showing a map for calculating a quantity of heatgenerated by the engine QENG from an intake air amount GA;

FIG. 16a is a table showing a map for calculating a correction factorKQTHA from an intake air temperature THA;

FIG. 16b is a table showing a map for calculating a correction factorKQSPD from a vehicle speed SPD;

FIG. 16c is a table showing a map for calculating a correction factorKQELB from a blower fan operating state ELB;

FIG. 17 is a flow chart showing a flow of processing steps of aclosure-malfunction flag setting program;

FIG. 18 is a time chart showing a method for diagnosing aclosure-malfunction (2);

FIG. 19 is a flow chart showing a flow of processing steps of anopen-malfunction diagnosing program for diagnosing theclosure-malfunction (2);

FIG. 20 is a flow chart showing a flow of processing steps of a coolanttemperature variation calculating program;

FIG. 21 is a block diagram showing the structure of the whole enginecooling system according to a second embodiment of the presentinvention;

FIG. 22 is a flow chart showing a flow of processing steps of a mainprogram for diagnosing a malfunction of a thermostat;

FIG. 23 is a flow chart showing a flow of processing steps of anopen-malfunction diagnosing program;

FIG. 24 is a flow chart showing a flow of processing steps of aclosure-malfunction diagnosing program;

FIG. 25 is a time chart showing the behavior of coolant temperature onthe engine side when the open-malfunction during which the thermostat iskept opened occurs as compared to the case when the thermostat operatesnormally;

FIG. 26 is a time chart showing the behavior of coolant temperature onthe radiator side when the open-malfunction during which the thermostatis kept opened occurs as compared to the case when the thermostatoperates normally;

FIG. 27 is a time chart showing the behavior of coolant temperature onthe engine side when the closure-malfunction during which the thermostatis kept opened occurs as compared with the case when the thermostatoperates normally;

FIG. 28 is a time chart showing the behavior of coolant temperature onthe radiator side when the closure-malfunction during which thethermostat is kept opened occurs as compared with the case when thethermostat operates normally;

FIG. 29 is a flow chart showing a flow of processing steps of anopen-malfunction diagnosing program according to a modification of thesecond embodiment;

FIG. 30 is a flow chart showing a flow of processing steps of anclosure-malfunction diagnosing program according to a modification ofthe second embodiment;

FIG. 31 is a block diagram showing the structure of the whole enginecooling system according to a third embodiment of the present invention;

FIG. 32 is a flow chart showing a flow of processing steps of a routinefor diagnosing an open-malfunction of a thermostat;

FIG. 33 is a flow chart showing a flow of processing steps of a routinefor setting a warm-up completion flag;

FIG. 34 is a time chart shoring a behavior of operation for diagnosingthe open-malfunction of the thermostat;

FIG. 35 is a flow chart showing a flow of processing steps of a routinefor diagnosing a malfunction of the thermostat according to amodification of the third embodiment;

FIG. 36 is a flow chart showing a flow of processing steps of a low loaddetermining routine;

FIG. 37 is a flow chart showing a flow of processing steps of a highload determining routine;

FIG. 38 is a flow chart showing a flow of processing steps of a targetthrottle opening computing routine;

FIG. 39 is a time chart showing a behavior when the closure-malfunctionoccurs; and

FIG. 40 is a time chart showing a behavior when the open-malfunctionoccurs.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

First Embodiment

In a cooling system of an engine shown in FIG. 1, a water jacket 12 isprovided within a cylinder block and a cylinder head of an engine 11 anda coolant (cooling water) is supplied within the water jacket 12. Athermostat 13 is provided at the outlet part of the water jacket 12 sothat the high temperature coolant which passes through the thermostat 13is sent to a radiator 15 via a coolant circulating path 14. The coolantwhose heat has been radiated by the radiator 15 and whose temperaturehas dropped is returned to the water jacket 12 via a coolant circulatingpath 16. Accordingly, when a valve within the thermostat 13 is opened,the coolant circulates through the path of the water jacket 12, thethermostat 13, the coolant circulating path 14, the radiator 15, thecoolant circulating path 16 and the water jacket 12 to cool and maintainthe engine 11 to a required temperature.

A water pump 17 is provided at the inlet of the water jacket 12. It isrotationally driven by the power of the engine transmitted via a belt 19to forcibly circulate the coolant within the coolant circulating paths.A radiator fan 18, i.e. an electrically driven fan, is provided behindthe radiator 15 to enhance the heat radiating effect of the radiator 15and to promote the cooling of the coolant within the radiator 15.

A coolant temperature sensor 20 for detecting the temperature of thecoolant within the water jacket 12 (coolant temperature on the engineside) which is the coolant circulating path on the side of the engine 11rather than the thermostat 13 is provided in the cylinder block of theengine 11. It is noted that the coolant temperature sensor 20 may be setat any position as long as it is on the coolant circulating path on theside of the engine 11 rather than the thermostat 13. That is, it may beset at the cylinder head side of the water jacket 12 for example.

An output signal of the coolant temperature sensor 20 is applied to anelectronic control unit (ECU) 22. The ECU 22 mainly comprised of amicrocomputer controls the engine 11 and diagnoses the malfunction ofthe thermostat 13. It is to be noted that the ECU 22 may be comprised oftwo ECUs separated as an engine control ECU and a thermostat malfunctiondiagnosis ECU or may be arranged so as to control the engine and todiagnose the malfunction of the thermostat 13 by one ECU.

In addition to the coolant temperature signal from the coolanttemperature sensor 20, the ECU 22 receives an engine speed signal froman engine speed sensor 23, an intake air amount signal from an intakeair sensor 24, an intake air temperature signal from an intake airtemperature sensor 25, a vehicle speed signal from a vehicle speedsensor 26 and a signal indicating an operating state of a blower motor(not shown) of an air conditioner 27 as information for controlling theengine 11 and to diagnose the malfunction of the thermostat 13. The ECU22 is connected to an alarm lamp 28 for alarming a malfunction of thethermostat 13 and to a backup RAM 29 which is a rewritable non-volatilememory for storing information of the malfunction of the thermostat 13and the like. The backup RAM 29 is arranged such that electric power issupplied from a battery not shown even when the engine is stopped tokeep the memory of the information on malfunction to allow theinformation on malfunction to be read out during repair and inspection.

Each program for diagnosing the malfunction of the thermostat is storedin a ROM (memory) built in the ECU 22. The thermostat 13 is diagnosedwhether it has the open-malfunction or the closure-malfunction byperforming those programs.

At this time, the thermostat 13 is diagnosed whether it has theopen-malfunction during which the thermostat 13 is kept opened by eitherone of the following five diagnosing methods (1) through (5).

(1) First Diagnosis of Open-Malfunction

At first, the first diagnosis of the open-malfunction will be explainedwith reference to FIG. 2 showing the behavior of the engine side coolanttemperature after starting the engine when the open-malfunction of thethermostat 13 has occurred as compared with the case when the thermostat13 operates normally. At the time of cold start when the engine 11 isstarted while it is cold, the engine side coolant temperature starts torise quickly right after the start of the engine as shown by thedot-chain line because the valve in the thermostat is closed and thecoolant is stopped from circulating when the thermostat 13 operatesnormally. However, the cold coolant within the radiator 15 is circulatedthrough the engine 11 from the beginning of the start of the engine evenduring the cold start when the open-malfunction occurs, so that theengine side coolant temperature drops temporarily as shown by the solidline because the cold coolant on the radiator 15 side flows in rightafter the start of the engine even during the cold start. The engineside coolant temperature then rises up moderately. The temporary drop ofthe engine side coolant temperature right after the start of the enginewhich occurs during the open-malfunction is a phenomenon which occursbecause the radiator side coolant temperature is lower than the engineside coolant temperature because the radiator is exposed to the outsidecold air while the engine is kept stopped.

Based on this temporary drop of the engine side coolant temperatureright after the start of the engine which occurs upon theopen-malfunction, the first diagnosis of the open-malfunction isimplemented whether the open-malfunction has occurred or not bydetermining the degree of drop of the engine side coolant temperatureright after the start of the engine and by comparing the degree of dropwith a reference value (reference). This open-malfunction detectingprogram shown in FIG. 3 is initiated per every predetermined time orevery predetermined crank angle rotation after when an ignition key (IGkey) is turned on.

When this program is initiated, it is determined at first in Step 101whether or not the IG key is ON and the engine is not started yet. Whenthe engine is not started yet, the process advances to Step 102 to storethe engine side coolant temperature detected by the coolant temperaturesensor 20 as initial values of a starting time coolant temperature THWSand a lowest coolant temperature THWmin. Then, the process advances toStep 103 to turn on a starter (not shown) to start the engine 11.

After that, it is determined in Step 104 whether it is the cold start ornot by determining whether the starting time coolant temperature THWS islower than a predetermined temperature thws which is set to be lowerthan a valve closing temperature of the thermostat 13. When it is notthe cold start, the program is finished without implementing thediagnosing processes thereafter.

When it is the cold start on the other hand, the lowest coolanttemperature THWmin is updated every time when the engine side coolanttemperature THW drops during the period from the start until when apredetermined time elapses by the processes in Steps 105 through 108.The process advances to Step 109 at the time when the predetermined timehas elapsed since the start of the engine to subtract the lowest coolanttemperature THWmin up to then from the starting time coolant temperatureTHWS to find a decrease of engine side coolant temperature ΔTHW afterthe start of the engine.

After that, the decrease of engine side coolant temperature ΔTHW afterthe start is compared with the reference value Δthw in Step 110. If thedecrease of engine side coolant temperature ΔTHW is greater than thereference value, the process advances to Step 111 to determine that thethermostat 13 has the open-malfunction. Then, this program ends bystoring the information on the open-malfunction in the backup RAM 29 inStep 112 and by lighting or flashing the alarm lamp 28 to warn a driverin Step 113. It is noted that when the decrease of engine side coolanttemperature ΔTHW is determined to be less than the reference value Δthwin Step 110, this program ends by determining that no open-malfunctionexists.

Although the diagnosis of the open-malfunction has been implemented bythe decrease of engine side coolant temperature ΔTHW after the start ofthis program, it may be implemented by the rate of drop of the engineside coolant temperature after the start (the rise of coolanttemperature per predetermined time, the rise of coolant temperature perpredetermined number of times of ignition or the rise of coolanttemperature per predetermined quantity of heat generated by the engine).

(2) Second Diagnosis of Open-Malfunction

As understood from FIG. 4, when the open-malfunction occurs, the coldcoolant within the radiator 15 is circulated through the engine 11 fromthe beginning of the start of the engine 11 even during the cold start,so that the engine side coolant temperature rises considerablymoderately as compared to the case when the thermostat 13 operatesnormally.

Based on this characteristic, the second diagnosis of theopen-malfunction is implemented by determining the degree of rise of theengine side coolant temperature within a predetermined time after thestart of the engine by an open-malfunction diagnosing program shown inFIG. 5.

When this program is initiated, the engine 11 is started after readingthe coolant temperature at the time of starting similarly to the firstdiagnosis of the open-malfunction (1) in Steps 121 through 124 and whenit is the case of the cold start, the processing steps after Step 125are executed as follows. At first, a time during which an idling stateis continuing from the cold start is accumulated by a post-starttemporal timer in Steps 125 and 126. When the accumulated time becomesequal to the predetermined time to (in Step 127), the increase of engineside coolant temperature ΔTHW within the predetermined time to after thecold start is calculated by subtracting the coolant temperature at thestarting time from the present engine side coolant temperature in Step128.

When the idling state does not continue until the predetermined time toelapses after the cold start, i.e. when it is determined to be NO inStep 125, the program ends without implementing the diagnosticprocessing steps on and after Step 126. It is because the quantity ofheat generated by engine fluctuates within the predetermined time andthe increase of engine side coolant temperature ΔTHW fluctuates when theidling state does not continue for the predetermined time to.

When the idling state continues for the predetermined time to after thecold start, the increase of engine side coolant temperature ΔTHWcalculated in Step 128 is compared with the reference value ΔTHW in Step129. When the increase of engine side coolant temperature ΔTHW isgreater than the reference value, i.e. when the speed of increase of theengine side coolant temperature is fast, the process advances to Step130 to determine that the thermostat 13 is closed normally and ends theprogram.

When the increase of engine side coolant temperature ΔTHW is determinedto be less than the reference value Δthw in Step 129 on the other hand,i.e. when the speed of increase of the engine side coolant temperatureis slow, the process advances to Step 131 to determine that thethermostat 13 has the open-malfunction. Then, the program ends bystoring the information on the open-malfunction in the backup RAM 29 inStep 132 and by lighting or flashing the alarm lamp 28 to warn thedriver of that in Step 133.

It is noted that the open-malfunction has been diagnosed while theidling state continues in consideration of that the engine operatingstate may influence on the behavior of the engine side coolanttemperature in the program, the open-malfunction may be diagnosed evenin the operating state other than the idling state if there is a periodduring which the operating state is continuously almost constant.

(3) Third Diagnosis of Open-Malfunction

It is arranged so that the diagnosis would not be influenced by thefluctuation of the engine operating state in the second diagnosis ofopen-malfunction (2) by calculating the increase of engine side coolanttemperature ΔTHW when the idling state is continuing for thepredetermined time from the cold start in order to calculate theincrease of coolant temperature ΔTHW within the predetermined time toafter the cold start. Accordingly, the open-malfunction cannot bediagnosed unless the idling state continues for the predetermined timeto from the cold start in the second diagnosis of the open-malfunction(2).

Therefore, in this third diagnosis of the open-malfunction (3), theinfluence of the fluctuation of the increase of the engine side coolanttemperature caused by the fluctuation of the engine operating state iseliminated in order to be able to diagnose the open-malfunctionaccurately even when the idling state does not continue from the coldstart by accumulating the quantity of heat generated by engine after thecold start and by calculating the increase of the engine side coolanttemperature during the period until when the accumulated value reaches apredetermined value.

In this malfunction diagnosing program shown in FIG. 6 for implementingthe third diagnosis of the open-malfunction, processing steps of thisprogram are the same as those of the program shown in FIG. 5 and used inthe second diagnosis of the open-malfunction (2) except for Steps 125 athrough 127 a related to the calculation of the increase of the engineside coolant temperature.

After reading the engine speed NE and the intake air amount GA duringthe cold start in Step 125 a, the quantity of heat Q generated by theengine 11 is calculated for the existing engine speed NE and the engineload GA/NE in Step 126 a from a two-dimensional map of the quantity ofheat Q parameterized by the engine speed NE and the load GA/NE. Then,the accumulated value of quantity of heat generated by engine ΣQ(i) isupdated by accumulating the latest quantity of heat generated by engineQ to the previously accumulated value of the quantity of heat generatedby engine ΣQ(i−1) in Step 126 b and it is determined whether or not theaccumulated value of quantity of heat generated by engine ΣQ(i) up tothis time has reached the predetermined value Σq(i) or not in Step 127a.

When the accumulated value of quantity of heat generated by engine ΣQ(i)after the cold start has reached the predetermined value Σq(i), theprocess advances to Step 128 to calculate the increase of engine sidecoolant temperature ΔTHW after the cold start by subtracting the coolanttemperature at the starting time from the present engine side coolanttemperature. The processes thereafter are the same as those in thesecond diagnosis of the open-malfunction (2).

By calculating the increase of engine side coolant temperature ΔTHWuntil when the accumulated value of quantity of heat generated by engineΣQ(i) after the cold start reaches the predetermined value to diagnosisof the open-malfunction, the influence of the fluctuation of theincrease of the engine side coolant temperature caused by thefluctuation of the engine operating state may be eliminated, allowingthe accuracy in diagnosing the open-malfunction to be improved.

It is noted that the open-malfunction may be diagnosed by accumulating anumber of times of ignition, instead of the quantity of heat Q generatedby engine 11, and by calculating the increase of the engine side coolanttemperature until when the accumulated value reaches the predeterminedvalue. The influence of the fluctuation of the increase of the engineside coolant temperature caused by the fluctuation of the engineoperating state may be reduced also in this case, allowing the accuracyin diagnosing the open-malfunction to be improved.

(4) Fourth Diagnosis of Open-Malfunction

In the fourth diagnosis of the open-malfunction shown in FIG. 8, thestarting time coolant temperature THWS is read before the start in Steps141 and 142. Then, after calculating a reference value K for determiningthe open-malfunction corresponding to the starting time coolanttemperature THWS by a preset map or equation in Step 143, the engine 11is started in Step 144. Then, in case of the cold start, the time duringwhich the idling state continues from the start is accumulated by apost-start temporal timer in Step 145 through 147. The accumulatingoperation of the post-start temporal timer is continued until when theengine side coolant temperature THW detected by the coolant temperaturesensor 20 rises up to a predetermined temperature thw in Step 148.

When the accelerator or throttle valve is operated to terminate theidling state before the engine side coolant temperature THW rises to thepredetermined temperature thw (when it is determined to be NO in Step146), this program ends without implementing the diagnostic processesthereafter. It is because the quantity of heat generated by enginefluctuates and the increase of the engine side coolant temperature THWfluctuates when the idling state is terminated.

Then, when the idling state continues until when the engine side coolanttemperature THW rises up to the predetermined temperature thw after thecold start, the accumulated time of the post-start temporal timer, i.e.the time required for the engine side coolant temperature THW to risesup to the predetermined temperature thw from the cold start, is comparedwith the reference value K calculated in Step 143. When this time isshorter than the reference value K, i.e. when the speed of increase ofthe engine side coolant temperature is fast, the process advances toStep 150 to determine that the thermostat 13 is closed normally and toend the program.

When it is determined in Step 149 that the time required for the engineside coolant temperature THW to rise up to the predetermined temperaturethw is greater than the reference value K on the other hand, i.e. whenthe speed of increase of the engine side coolant temperature is slow,the process advances to Step 151 to determine that the thermostat 13 hasthe open-malfunction. Then, the program ends after storing theinformation on the open-malfunction in the backup RAM 29 in Step 152 andlighting or flashing the alarm lamp 28 to warn the driver of that inStep 153.

The reference value K for determining the open-malfunction correspondingto the starting time coolant temperature THWS is calculated in Step 143in this program in consideration of that the time required for theengine side coolant temperature THW to rise up to the predeterminedtemperature after the cold start differs depending on the starting timecoolant temperature THWS. Thereby, the open-malfunction may be diagnosedreliably without being influenced by the starting time coolanttemperature THWS.

It is noted that the open-malfunction may be diagnosed by accumulatingthe time until when the increase of the engine side coolant temperatureTHW after the start reaches the predetermined value instead ofaccumulating the time until when the engine side coolant temperature THWrises up to the predetermined temperature. This method has a merit thatthe influence of the starting time coolant temperature THWS given to theaccumulated time is lessened.

Further, the object of the accumulation may be changed from the elapsedtime from the start to the quantity of heat generated by engine or thenumber of times of ignition. When the quantity of heat generated byengine is to be accumulated, it may be achieved by implementing the sameprocedure from Steps 125 a through 126 b shown in FIG. 6. Then, theopen-malfunction may be diagnosed by comparing the accumulated value ofthe quantity of heat generated by engine (or number of times ofignition) until when the engine side coolant temperature reaches thepredetermined temperature after the cold start or until when theincrease of the engine side coolant temperature reaches thepredetermined degree with the reference value. Thereby, the influence ofthe fluctuation of the engine side coolant temperature caused by thefluctuation of the engine operating state may be eliminated and theopen-malfunction may be diagnosed accurately even if the idling statedoes not continue.

(5) Fifth Diagnosis of Open-Malfunction

In this fifth diagnosis, the increase of the engine side coolanttemperature ΔTHW is determined per predetermined time after the start ofthe engine as shown in FIG. 9 to diagnose the open-malfunction based onthe number of times when the increase of the engine side coolanttemperature drops below a reference value. This diagnosing program isshown in FIGS. 10 and 11.

When this program is initiated, the engine 11 is started after readingthe starting time coolant temperature in Steps 161 through 164 similarlyto the first diagnosis of the open-malfunction. In case of the coldstart, the processing steps after Step 165 are executed as follows. Atfirst, a virtual or provisional fail counter is cleared in Step 165.Then, processes for calculating the increase of engine side coolanttemperature ΔTHW within a predetermined time per predetermined time arerepeated until when the engine side coolant temperature THW reaches thevalve opening temperature of the thermostat 13 in the ensuing Steps 166through 171.

That is, when the engine side coolant temperature THW is lower than thevalve opening temperature of the thermostat 13, the quantity of heatgenerated by engine Q is calculated from the two-dimensional map fromthe engine speed NE and the intake air amount GA (load GA/NE) and theaccumulated value of quantity of heat generated by engine ΣQ(i) isupdated by accumulating the quantity of heat generated by engine Q ofthis time to the previously accumulated value of the quantity of heatgenerated by engine ΣQ(i−1) by the processes in Steps 167 through 169.This accumulated value of quantity of heat generated by engine ΣQ isused in calculating the reference value of the open-malfunction.

Then, the engine side coolant temperature THW at each moment is storedas the present coolant temperature THWF(i) in Step 171 every time whenthe predetermined time elapses and the increase of coolant temperatureΔTHW per predetermined time is calculated by subtracting the previouscoolant temperature THWF(i−1) from the present coolant temperatureTHWF(i) in Step 172.

After that, the reference value Σq corresponding to the accumulatedvalue of quantity of heat generated by engine ΣQ(i) within thepredetermined time calculated in Step 169 is calculated by the map orexpression set in advance in Step 173. Thereby, the reference value Σq,in which the influence of the fluctuation of the increase of the engineside coolant temperature caused by the fluctuation of the engineoperating state is taken into consideration, is calculated. Aftercalculating the reference value, the accumulated value of quantity ofheat generated by engine ΣQ(i) is cleared. Then, the increase of coolanttemperature ΔTHW per predetermined time is compared with the referencevalue calculated in Step 173. When the increase of coolant temperatureΔTHW is less than the reference value, there is a possibility of theopen-malfunction, so that the process advances to Step 175 to incrementthe virtual fail counter and ends the program. It is noted that when theincrease of coolant temperature ΔTHW per predetermined time is greaterthan the reference value, the program is finished without doinganything.

Thus, the processes of calculating the increase of coolant temperatureΔTHW per predetermined time to compare with the reference value and ofincrementing the virtual fail counter when the ΔTHW≧reference value arerepeated until when the engine side coolant temperature THW reaches thevalve opening temperature of the thermostat 13. When the engine sidecoolant temperature THW reaches the valve opening temperature, theabove-mentioned process is finished. Then, the process advances to Step176 to compare the value of the virtual fail counter with apredetermined value. When the value of the virtual fail counter isgreater than the predetermined value, the process advances to Step 177to determine that the thermostat 13 has the open-malfunction. Then, theprogram ends after storing the information on the open-malfunction inthe backup RAM 29 in Step 178 and lighting or flashing the alarm lamp 28to warn the driver of that in Step 179. It is noted that when it isdetermined that the value of the virtual fail counter is smaller thanthe predetermined value in Step 176, it is not determined to be theopen-malfunction and the program ends.

Although the increase of coolant temperature ΔTHW per predetermined timehas been calculated in this program, the increase of temperature perpredetermined quantity of heat generated by engine or the increase oftemperature per predetermined number of times of ignition may becalculated to compare with the reference value. In short, the thermostat13 may be diagnosed whether it has the open-malfunction or not byperiodically determining the increase of the engine side coolanttemperature after the start of the engine and based on the number oftimes when the increase of the engine side coolant temperature is lessthan the reference value. Thereby, the open-malfunction may be diagnosedrepeatedly based on the increase of the engine side coolant temperatureand the open-malfunction may be diagnosed reliably.

While the thermostat 13 has been diagnosed whether it has theopen-malfunction or not during idling (or during the period in which thealmost constant operating state continues) by considering that thebehavior of the engine side coolant temperature is influenced by theengine operating state and the behavior of the engine side coolanttemperature has been determined based on the quantity of heat generatedby engine or the number of times of ignition in each of the firstthrough fifth diagnosis of the open-malfunction, the behavior of theengine side coolant temperature is influenced not only by the engineoperating state but also by the factors such as the vehicle speed,outside temperature, intake air temperature and operating state of theair conditioner which influence the radiation of the coolant.Accordingly, the data such as the reference value, predetermined periodand detected coolant temperature used in the diagnosis of theopen-malfunction may be corrected based at least on one of the vehiclespeed, outside temperature, intake air temperature and operating stateof the air conditioner. Thereby, the open-malfunction may be diagnosedwhile taking the radiation of the coolant into consideration and theaccuracy in diagnosing the open-malfunction may be improved that much.

Further, because no heat is generated by the engine when fuel is cutoff, the elapsed time, the number of times of ignition and the quantityof heat generated by engine may be accumulated except for the periodduring which the fuel is cut off.

The diagnosis of the closure-malfunction during which the thermostat 13is kept closed is implemented by either one of the following twomethods.

(1) First Diagnosis of Closure-Malfunction

The first diagnosis of the closure-malfunction is made based on thebehavior of the engine side coolant temperature when theclosure-malfunction occurs in comparison with the case when thethermostat operates normally. As shown in FIG. 12, when the engine sidecoolant temperature exceeds a thermostat valve opening temperature, thevalve of the thermostat 13 is opened when it is normal and the coldcoolant on the radiator 15 side is circulated to the engine 11, thusdropping the engine side coolant temperature. When the thermostat 13 hasthe closure-malfunction on the other hand, the valve of the thermostat13 is not opened, no coolant is circulated and the engine side coolanttemperature continues to rise up.

The thermostat 13 is diagnosed whether it has the closure-malfunction ornot by comparing the rate of change of the engine side coolanttemperature with a reference value after when the engine side coolanttemperature reaches the thermostat valve opening temperature. Here, therate of change of the engine side coolant temperature may be determinedby any one of the variation of coolant temperature per predeterminedtime, the variation of coolant temperature per predetermined number oftimes of ignition and the variation of coolant temperature perpredetermined quantity of heat generated by engine.

The processing steps of the closure-malfunction diagnosing program isshown in FIG. 13 and is initiated per every predetermined time (e.g. per200 ms) after when the IG key has been turned on.

When the program is initiated, the engine side coolant temperature THWdetected by the coolant temperature sensor 20 is read in Step 201. Then,it is determined whether the sensors (the coolant temperature sensor 20,the intake air amount sensor 24, the intake air temperature sensor 25and the vehicle speed sensor 26) used in the diagnosis of theclosure-malfunction are normal or not in Step 202. The determinationwhether those sensors are normal or not is made by determining whetherthe output voltage of the sensors are within a predetermined voltagerange or not. When all the relevant sensors are determined to beabnormal, the program ends without implementing the processes thereafterbecause the diagnosis of the malfunction cannot be carried out normally.

When the sensors are normal, the process advances to Step 203 todetermine whether misfire has occurred or not. When the misfire occurs,the quantity of heat generated by engine drops and the behavior of theengine side coolant temperature fluctuates, so that the program endswith implementing the processes thereafter.

When no misfire has occurred, the process advances to Step 204 todetermine whether it is the cold start or not by determining whether theengine side coolant temperature THWS at the time of start is lower than60° C. or not (the predetermined temperature less than the valve closingtemperature of the thermostat 13). When it is not the cold start, theprogram ends without implementing the processes thereafter.

When it is the cold start, the process advances to Step 205 to determinewhether the accumulated value of quantity of heat generated by engineSQENG accumulated by a program for accumulating quantity of heatgenerated by engine described later with reference to FIG. 14 hasreached the reference quantity of heat (sqeng) or not. The referencequantity of heat is the quantity of heat generated by engine necessaryfor the normal thermostat 13 to open the valve reliably after the coldstart. Accordingly, when the accumulated value of quantity of heatgenerated by engine SQENG has not reached the reference quantity ofheat, the program ends without implementing the processes thereafter.

When the accumulated value of quantity of heat generated by engine SQENGhas reached the reference quantity of heat on the other hand, theprocess advances to Step 206 to determine whether or not aclosure-malfunction flag XDTHWCL which is set by a closure-malfunctionflag setting program described later with reference to FIG. 16 is “0”,meaning the closure-malfunction. It is noted that theclosure-malfunction flag XDTHWCL is set at “1” which means normal duringthe initialization.

When the closure-malfunction flag XDTHWCL is “0” meaning theclosure-malfunction, the process advances to Step 207 to determine thatthe thermostat 13 has the closure-malfunction. Then, the program endsafter storing the information on the closure-malfunction (engine speed,intake air amount, engine side coolant temperature, vehicle speed and amalfunction mode at the time of the closure-malfunction) in the backupRAM 29 in Step 208 and lighting or flashing the alarm lamp 28 to warnthe driver of that in Step 209.

Accumulation of heat generated by engine is attained by the programshown in FIG. 14 and is initiated per predetermined time (e.g. per 100ms) after when the IG key has been turned on and accumulates thequantity of heat generated by engine after the start as follows. Atfirst, an intake air mount GA, an intake air temperature THA, a vehiclespeed SPD and an operating state ELB of a blower fan of the airconditioner 27 are read in Step 221. It is then determined in Step 222whether fuel is being cut off or not. The quantity of heat generated byengine becomes zero and the engine side coolant temperature drops due tothe radiation during when the fuel is cut off. Accordingly, the processadvances to Step 226 when the fuel is being cut off to subtract apredetermined value (e.g. 10) from the previously accumulated value ofthe quantity of heat generated by engine SQENG(i−1) to cancel theinfluence of the fuel cut off.

When the fuel is not being cut off on the other hand, the processadvances to Step 223 to calculate the quantity of heat generated byengine QENG in response to the intake air amount GA from a map shown inFIG. 15. It is noted that the intake air pressure or fuel injectionamount may be used instead of the intake air amount GA as the parameterfor calculating the quantity of heat generated by engine QENG.

After calculating the quantity of heat generated by engine QENG, theprocess advances to Step 224 to accumulate the quantity of heatgenerated by engine QENG of this time to the previously accumulatedvalue of the quantity of heat generated by engine SQENG(i−1) to updatethe accumulated value of quantity of heat generated by engine SQENG(i).After that, the accumulated value of quantity of heat generated byengine SQENG(i) is corrected by multiplying correction factors KQTHA,KQSPD and KQEPB corresponding to the intake air temperature THA, thevehicle speed SPD and the operating state of the blower fan of the airconditioner 27 ELB in Step 225.

The correction factor KQTHA corresponding to the intake air temperatureTHA is calculated corresponding to the intake air amount GA from a mapshown in FIG. 16a. It is noted that the outside air temperature may beused instead of the intake air temperature THA. The correction factorKQSPD corresponding to the vehicle speed SPD is calculated correspondingto the vehicle speed SPD from a map shown in FIG. 16b. The correctionfactor KQELB corresponding to the operating state of the blower fan ELBis calculated corresponding to ON/OFF of the blower fan from a map shownin FIG. 16c.

The accumulated value of quantity of heat generated by engine SQENG(i)is corrected corresponding to the intake air temperature THA, thevehicle speed SPD and the operating state of the blower fan ELB, becausethe radiation of the coolant is influenced and the behavior of theengine side coolant temperature is fluctuated by all of the intake airtemperature THA, the vehicle speed SPD and the operating state of theblower fan ELB. It is noted that because the radiation of the coolantchanges depending on the operation modes of the blower fan (strong/weakblow, introduction of outside air or air is circulated within thecompartment), the correction factor KQELB may be changed depending onthe operation modes.

The closure-malfunction flag setting program is shown in FIG. 17 and isinitiated per predetermined time (e.g. per 100 ms) after when the IG keyhas been turned on and sets the closure-malfunction flag XDTHWCL asfollows. At first, the variation of the engine side coolant temperatureDTHW per predetermined time (e.g. per 100 ms) is calculated bysubtracting the engine side coolant temperature THW(i) of this time fromthe previous engine side coolant temperature THW(i−1) in Step 231.

After that, it is determined whether the electrically driven radiatorfan 18 is off or not in Step 232. When the radiator fan 18 is OFF, theprocess advances to Step 233 to determine whether a predetermined time(e.g. five seconds) has elapsed or not after when the radiator fan 18has been switched from ON to OFF. When this time has elapsed, theprocess advances to Step 234 to determine the closure-malfunction. Whenthe response of either one of the Steps 232 and 233 is “No”, i.e. theradiator fan 18 is ON or the predetermined time (e.g. five seconds) hasnot elapsed from when the radiator fan 18 is switched from ON to OFF,the program ends without determining the closure-malfunction so as notto be influenced by the heat radiation of the coolant caused by the flowof the radiator fan 18.

When the predetermined time (e.g. five seconds) has elapsed since whenthe radiator fan 18 has been turned off, the variation of the coolanttemperature DTHW is compared with the reference value dthw (e.g. 0° C.)in Step 234. When the variation of the coolant temperature DTHW issmaller than the reference value, the thermostat 13 is assumed to beopening normally, so that the program ends by advancing to Step 235 toset the closure-malfunction flag XDTHWCL at “1” indicating that thethermostat 13 is normal.

When the variation of coolant temperature DTHW is greater than thereference value dthw on the other hand, i.e. when the engine sidecoolant temperature THW continuously rises up abnormally, the processadvances to Step 236 to set the closure-malfunction flag XDTHWCL at “0”indicating the closure-malfunction and ends the program. It is notedthat the reference value which is compared with the variation of coolanttemperature DTHW in Step 234 is not limited only to 0° C. and may be aplus temperature.

(2) Second Diagnosis of Closure-Malfunction

While the variation of coolant temperature DTHW per predetermined timehas been calculated in the first diagnosis of the closure-malfunction,the variation of coolant temperature DTHWSQ per predetermined quantityof heat generated by engine is calculated in the second diagnosis ofclosure-malfunction. Further, while the closure-malfunction has beendiagnosed when the accumulated value of the quantity of heat generatedby engine after the start reached the predetermined value in the firstdiagnosis of closure-malfunction, the diagnosis of closure-malfunctionis implemented when the engine side coolant temperature exceeds thevalve opening temperature of the thermostat 13 by a predeterminedtemperature.

The second diagnosis of closure-malfunction is shown in FIG. 19 and isinitiated per every predetermined time (e.g. per 200 ms) after when theIG key has been turned.

When the program is initiated, the engine side coolant temperature THWdetected by the coolant temperature sensor 20 is read and the variationof coolant temperature DTHWSQ per predetermined quantity of heatgenerated by engine calculated by the coolant temperature variationcalculating program shown in FIG. 19 is read in Step 241. When it isdetermined that the sensors such as the coolant temperature sensor 20used in the diagnosis of closure-malfunction are normal and no misfirehas occurred in Steps 242 and 243, the process advances to Step 244 tocompare the engine side coolant temperature THW with a temperature suchas 95° C. which is higher than the valve opening temperature (e.g. 90°C.) of the thermostat 13 by a predetermined temperature (e.g. 5° C.).This temperature causes the thermostat 13 to open certainly if thethermostat 13 is normal. Accordingly, the program ends withoutimplementing the diagnostic processes thereafter when the engine sidecoolant temperature THW is less than 95° C.

When the engine side coolant temperature THW exceeds 95° C. on the otherhand, the process advances to Step 245 to compare the variation ofcoolant temperature DTHWSQ per predetermined quantity of heat generatedby engine with the reference value dthwsq (e.g. 0° C.). When thevariation of coolant temperature DTHWSQ is less than the referencevalue, the thermostat 13 is assumed to be opening normally, so that theprogram ends without implementing the processes thereafter.

When the variation of coolant temperature DTHWSQ is greater than thereference value, it means that the engine side coolant temperature THWis continuously rising up abnormally, so that the process advances toStep 246 to determine that the thermostat 13 has theclosure-malfunction. Then, the program ends after storing theinformation on the closure-malfunction in the backup RAM 29 in Step 247and lighting or flashing the alarm lamp 28 to warn the driver of that inStep 248.

The coolant temperature variation calculating program shown in FIG. 20is initiated per predetermined time (e.g. per 100 ms) after when the IGkey has been turned on and calculates the variation of coolanttemperature DTHWSQ per predetermined quantity of heat generated byengine as follows. At first, the accumulated value of quantity of heatgenerated by engine SQENG calculated by the program for accumulating thequantity of heat generated by engine described before with reference toFIG. 14 and the engine side coolant temperature THW are read in Step251.

After that, it is determined whether the accumulated value of quantityof heat generated by engine SQENG has exceeded the predetermined value(FIG. 17) or not. The variation DTHWSQ of the engine side coolanttemperature is calculated by subtracting the coolant temperature THW ofthis time from the previous coolant temperature THWO in Step 253 everytime when the accumulated value of quantity of heat generated by engineSQENG exceeds the predetermined value. After that, the program endsafter updating the previous coolant temperature THWO by the coolanttemperature THW of this time and clearing the accumulated value ofquantity of heat generated by engine SQENG.

It is noted that although the variation of coolant temperature DTHWSQper predetermined quantity of heat generated by engine has beencalculated in this program, the variation of coolant temperature perpredetermined number of times of ignition may be calculated. Further,the variation of coolant temperature per predetermined time may becalculated during the period in which the idling state continues or analmost constant operating state continues.

It is noted that although the radiator fan 18 has been composed of theelectrically driven fan in the example of the system structure of FIG.1, the radiator fan may be linked with the water pump 17 so that theradiator fan and the water pump 17 are driven together by the power ofthe engine. Further, the position where the thermostat 13 is mounted isnot limited only to the outlet part of the water jacket 12. It may bemounted at the inlet part or other parts of the water jacket 12.

The above first embodiment may be arranged such that only either one ofthe open-malfunction diagnosing program or the closure-malfunctiondiagnosing program is implemented.

Second Embodiment

A second embodiment of the present invention will be explained belowwith reference to FIGS. 21 through 28. While the cooling system of theengine 11 shown in FIG. 21 is similar to that of the first embodimentshown in FIG. 1, the water pump 17 is provided at the inlet of the waterjacket 12 and is linked with a cooling fan 18 provided behind theradiator 15 so that the water pump 17 and the radiator fan 18 are driventogether by engine power transmitted via the belt 19. The circulation ofthe coolant within the coolant circulating path is accelerated by therotation of the water pump 17 and the heat radiating effect of theradiator 15 is enhanced by the rotation of the cooling fan 18 toaccelerate the cooling of the coolant within the radiator 15.

In addition to the engine side coolant temperature sensor 20, a radiatorside coolant temperature sensor 21 for detecting the temperature of thecoolant (radiator side coolant temperature) supplied to the engine 11 isprovided on the way of the coolant circulating path 14 on the radiator15 side from the thermostat 13. It is noted that the position where theradiator side coolant temperature sensor 21 is mounted may be any placeon the coolant circulating path on the radiator 15 side from thethermostat 13 and may be provided on the radiator 15 for example.

Programs for diagnosing the thermostat malfunction shown in FIGS. 22through 24 are stored in a ROM built within the ECU 22. The thermostat13 is diagnosed whether it has the open-malfunction or theclosure-malfunction by executing those programs.

The malfunction diagnosing program for controlling the processes of thewhole diagnosis of the malfunction of the thermostat is repeatedlyactivated per predetermined time or per crank angle after when theignition switch not shown is turned on. When this program is initiated,an open-malfunction diagnosing program shown in FIG. 23 is executed inStep 2100 to diagnose whether the open-malfunction in which thethermostat 13 is kept opened occurred or not. After that, aclosure-malfunction diagnosing program shown in FIG. 24 is executed inStep 2200 to diagnose whether the closure-malfunction in which thethermostat 13 is kept closed occurred or not.

The behavior of the engine side coolant temperature Te and of theradiator side coolant temperature Tr when the open-malfunction hasoccurred as compared with those in the normal time are shown in FIGS. 25and 26. Because the thermostat 13 is closed when it is normal at thetime of cold start when the engine 11 is started while it is cold, thecoolant is stopped from circulating to accelerate the rise of the engineside coolant temperature. Thus, because the radiator side coolanttemperature rarely rises, the difference of temperature between theengine side coolant temperature and the radiator side coolanttemperature should normally increase as time elapses. When theopen-malfunction occurs on the other hand, the cold coolant within theradiator 15 is circulated to the water jacket 12 of the engine 11 fromthe beginning of the start even at the time of cold start, so that thedifference of temperature between the engine side coolant temperatureand the radiator side coolant temperature after the start isconsiderably small as compared with the case of the normal thermostat.

Based on this point, the thermostat 13 is determined whether it isnormally closed or has the open-malfunction depending on whether thedifference of temperature between the engine side coolant temperature Teand the radiator side coolant temperature Tr at a predetermined periodafter the cold start is large or not in the open-malfunction diagnosingprogram shown in FIG. 23. In more detail, it is determined in Step 2101whether it is the cold start or not by determining whether the engineside coolant temperature Te at the time of start is less than the valveclosing temperature of the thermostat 13. If it is not the cold start,the program ends without diagnosing the open-malfunction.

The diagnostic of the open-malfunction is implemented at the time ofcold start because the engine side coolant temperature Te and theradiator side coolant temperature Tr are almost equal or close to eachother and because the increase of the coolant temperature at the time ofopen-malfunction is largely different from that at the normal time inthe period during which the engine side coolant temperature Te reachesthe valve opening temperature of the thermostat 13 after the cold startand the open-malfunction may be readily detected as compared with otheroperating period.

When it is determined to be the cold start in Step 2101, the processadvances to Step 2102 to determine whether the open-malfunctiondiagnosing conditions hold or not. Here, the open-malfunction diagnosingconditions are (a) both the engine side coolant temperature sensor 20and the radiator side coolant temperature sensor 21 are normal, (b) apredetermined time has elapsed after the cold start (the predeterminedtime is set within the time T1 during which the engine side coolanttemperature Te reaches the valve opening temperature of the thermostat13 after the cold start), and (c) the engine side coolant temperature Teis lower than the valve opening temperature of the thermostat 13. Whenall of these conditions (a) through (c) are met, the open-malfunctiondiagnosing conditions hold.

Here, the condition (a), i.e., whether the both coolant temperaturesensors 20 and 21 are normal, is determined whether an output voltage ofthe coolant temperature sensors 20 and 21 falls within a predeterminedrange. The condition (b), i.e., whether the predetermined time haselapsed after the cold start, is a temporal condition necessary untilwhen a clear difference appears in the behavior of the coolanttemperature during the open-malfunction time and during the normal time.The condition (c), i.e., whether the engine side coolant temperature Teis lower than the valve opening temperature of the thermostat 13, is setbecause it becomes difficult to discriminate the open-malfunction whenthe engine side coolant temperature Te exceeds the valve openingtemperature of the thermostat 13 and the thermostat 13 is opened.

When conditions (a) through (c) are not met in Step 2102, theopen-malfunction diagnosing conditions do not hold and the program endswithout implementing the diagnosis of the open-malfunction.

When all the conditions (1) through (3) are met and the open-malfunctiondiagnosing conditions hold on the other hand, the process advances toStep 2103 to calculate a difference of temperature (Te−Tr) between theengine side coolant temperature Te and the radiator side coolanttemperature Tr. Then, an open-malfunction discriminating reference valueα for determining the open-malfunction from the difference oftemperature (Te−Tr) is calculated from a map or a mathematicalexpression by parameterizing at least one of an intake air amount GA, anengine speed Ne, an intake air temperature, a vehicle speed and theoperating state of the blower motor of the air-conditioner 27 which areparameters influencing the calorific heat value of the engine 11 and theradiation of the coolant.

After that, the difference of temperature (Te−Tr) between the engineside coolant temperature Te and the radiator side coolant temperature Tris compared with the open-malfunction discriminating reference value αin Step 2105. When the difference of temperature (Te−Tr) is greater thanthe open-malfunction discriminating reference value α, the processadvances to Step 2106 to determine that the thermostat 13 is normallyopened as it should be and then to end the program.

When the difference of temperature (Te−Tr) between the engine sidecoolant temperature Te and the radiator side coolant temperature Tr issmaller than the open-malfunction discriminating reference value α onthe other hand, the process advances to Step 2107 to determine that thethermostat 13 has the open-malfunction. The program ends after lightingor flashing the alarm lamp 28 in Step 2108 to warn the driver of thatand by storing the information on the open-malfunction in the backup RAM29.

The closure-malfunction diagnosing program shown in FIG. 24 is based onthe behavior of the engine side coolant temperature Te and the radiatorside coolant temperature Tr shown in FIGS. 27 and 28 when theclosure-malfunction by which the thermostat 13 is kept closed occurs ascompared with the case of the normal thermostat. When the engine sidecoolant temperature exceeds the thermostat valve opening temperature,the valve is opened when the thermostat 13 is normal and the coldcoolant on the radiator side circulates to the engine 11, thus droppingthe engine side coolant temperature and increasing the radiator sidecoolant temperature, so that the difference of temperature between theengine side coolant temperature and the radiator side coolanttemperature becomes small as time elapses. When the closure-malfunctionoccurs on the other hand, the thermostat 13 is not opened even when theengine side coolant temperature Te exceeds the thermostat valve openingtemperature, no coolant is circulated and the engine side coolanttemperature Te continuously rises up. However, because the radiator sidecoolant temperature Tr does not rise so much, the difference oftemperature between the engine side coolant temperature Te and theradiator side coolant temperature Tr becomes greater as time elapses.

Based on this point, the closure-malfunction diagnosing program shown inFIG. 24 determines the thermostat 13 whether it normally opens or hasthe closure-malfunction by determining whether the difference oftemperature between the engine side coolant temperature Te and theradiator side coolant temperature Tr is large or small within apredetermined period after when the engine side coolant temperaturereaches the valve opening temperature of the thermostat 13 at time T1from the cold start. In more detail, it is determined whether or not theengine side coolant temperature Te at the starting time is less than thevalve closing temperature of the thermostat 13 in Step 2201. When it isnot the cold start, the program ends without diagnosing theclosure-malfunction.

When it is determined to be cold start in Step 2201 on the other hand,the process advances to Step 2202 to determine whether or notclosure-malfunction diagnosing conditions hold. Here, theclosure-malfunction diagnosing conditions are (a) both the coolanttemperature sensor 20 and the radiator side coolant temperature sensor21 are normal, (b) a predetermined time has elapsed after when theengine side coolant temperature Te has exceeded the valve openingtemperature of the thermostat 13 and (c) the engine side coolanttemperature Te is higher than the valve closing temperature of thethermostat 13. When all of these conditions (a) through (c) are met, theclosure-malfunction diagnosing conditions hold.

Here, the condition (b) (i.e. whether or not the predetermined time haselapsed after exceeding the valve opening temperature of the thermostat13) is a temporal condition necessary until when a clear differenceappears in the behavior of the coolant temperatures Te and Tr during theclosure-malfunction time and during the normal time. The condition (c),i.e., whether the engine side coolant temperature Te is higher than thevalve closing temperature of the thermostat 13, is set because itbecomes difficult to discriminate the closure-malfunction when theengine side coolant temperature Te is less than the valve closingtemperature of the thermostat 13 and the thermostat 13 is closed.

When conditions (a) through (c) are not met in Step 2202, theclosure-malfunction diagnosing conditions do not hold and the programends without implementing the diagnosis of the closure-malfunction.

When all the conditions (a) through (c) are met and theclosure-malfunction diagnosing conditions hold on the other hand, theprocess advances to Step 2203 to calculate a difference of temperature(Te−Tr) between the engine side coolant temperature Te and the radiatorside coolant temperature Tr. Then, a closure-malfunction discriminatingreference value β for determining the closure-malfunction from thedifference of temperature (Te−Tr) is calculated from a map or amathematical expression by parameterizing at least one of the intake airamount GA, the engine speed Ne, the intake air temperature, the vehiclespeed and the operating state of the blower motor of the air-conditioner27 which are parameters influencing the calorific heat value of theengine 11 and the radiation of the coolant.

After that, the difference of temperature (Te−Tr) between the engineside coolant temperature Te and the radiator side coolant temperature Tris compared with the closure-malfunction discriminating reference valueβ in Step 2205. When the difference of temperature (Te−Tr) is less thanthe closure-malfunction discriminating reference value β, the processadvances to Step 2206 to determine that the thermostat 13 is normallyclosed as it should be and then to end the program.

When the difference of temperature (Te−Tr) between the engine sidecoolant temperature Te and the radiator side coolant temperature Tr islarger than the closure-malfunction discriminating reference value β onthe other hand, the process advances to Step 2207 to determine that thethermostat 13 has the closure-malfunction. The program ends afterlighting or flashing the alarm lamp 28 in Step 2208 to warn the driverof that and by storing the information on the closure-malfunction in thebackup RAM 29.

According to the second embodiment, the malfunction of the thermostat 13can be detected based on the engine side coolant temperature Te and theradiator side coolant temperature Tr detected by the engine side coolanttemperature sensor 20 and the radiator side coolant temperature sensor21, so that the aggravation of fuel consumption, the increase of noxiousexhaust emission and the overheat caused by the malfunction of thethermostat 13 may be prevented beforehand. Still more, because thecoolant temperature sensor for controlling the engine which is providedin the conventional engine may be used as the coolant temperature sensor20, the system may be relatively simply constructed just by adding theradiator side coolant temperature sensor 21 anew to the conventionalengine control system and the increase of the cost is minimized, thussatisfying the demand of reducing the cost.

Further, because the closure-malfunction discriminating reference valueis calculated by parameterizing at least one of the intake air amountGA, the engine speed Ne, the intake air temperature, the vehicle speedand the operating state of the blower motor of the air-conditioner 27which are the parameters influencing the calorific value of the engine11 and the radiation of the coolant, it becomes possible to determinethe malfunction while considering the calorific value of the engine 11and the radiation of the coolant and thereby the accuracy in diagnosingthe malfunction may be enhanced.

Modifications of Second Embodiment

Alternatively to the second embodiment, the diagnosis whether thethermostat 13 has the open-malfunction/closure-malfunction isimplemented based on the rate of change of temperature of the engineside coolant temperature Te and the radiator side coolant temperature Trin this modification shown in FIGS. 29 and 30.

Similarly to the case shown in FIG. 23, the open-malfunction diagnosingprogram shown in FIG. 29 executes the processes for diagnosing theopen-malfunction on and after Step 2103 a when it is the cold start andthe open-malfunction diagnosing conditions hold in Steps 2101 and 2102.The open-malfunction diagnosing conditions are the same as those in thesecond embodiment. In diagnosing the open-malfunction, the rate ofchange of engine side coolant temperature ΔTe calculated from anabsolute value of the difference between the previous engine sidecoolant temperature Te(i−1) and the current engine side coolanttemperature Te(i) and the rate of change of radiator side coolanttemperature ΔTr is calculated from the absolute value of the differencebetween the previous radiator side coolant temperature Tr(i−1) and thecurrent radiator side coolant temperature Tr(i) in Step 2103 a.

After that, an open-malfunction discriminating reference value γ fordetermining the open-malfunction from the rate of change of engine sidecoolant temperature ΔTe and an open-malfunction discriminating referencevalue δ for determining the open-malfunction from the rate of change ofradiator side coolant temperature ΔTr are calculated from a map or amathematical expression by parameterizing at least one of the intake airamount GA, the engine speed Ne, the intake air temperature, the vehiclespeed and the operating state of the blower motor of the air-conditioner27 which are the parameters influencing the calorific heat value of theengine 11 and the radiation of the coolant in Step 2104 a.

Then, it is determined whether or not the rate of change of engine sidecoolant temperature ΔTe is larger than the open-malfunctiondiscriminating reference value γ and the rate of change of radiator sidecoolant temperature ΔTr is less than the open-malfunction discriminatingreference value δ in Step 2105 a. When the both conditions of ΔTe≧γ andΔTr≦δ are met, the process advances to Step 2106 to determine that thethermostat 13 is normally opened and to end the program.

When even one of the both conditions of ΔTe≧γ and ΔTr≦δ are not met onthe other hand, the process advances to Step 2107 to determine that thethermostat 13 has the open-malfunction and ends the program afterlighting or flashing the alarm lamp 28 to warn the driver of that inStep 2108 and storing the information on the open-malfunction in thebackup RAM 29.

Meanwhile, the closure-malfunction diagnosing program shown in FIG. 30executes, similarly to the case shown in FIG. 24, the processes fordiagnosing the closure-malfunction on and after Step 2203 a when it isthe cold start and the closure-malfunction diagnosing conditions hold inSteps 2201 and 2202. The closure-malfunction diagnosing conditions arethe same as those in the second embodiment. In diagnosing theclosure-malfunction, the rate of change of engine side coolanttemperature ΔTe is calculated from an absolute value of the differencebetween the previous engine side coolant temperature Te(i−1) and theengine side coolant temperature Te(i) of this time and the rate ofchange of radiator side coolant temperature ΔTr is calculated from theabsolute value of the difference between the previous radiator sidecoolant temperature Tr(i−1) and the current radiator side coolanttemperature Tr(i) in Step 2203 a.

After that, a closure-malfunction discriminating reference value ε fordetermining the closure-malfunction from the rate ΔTe/ΔTr of the rate ofchange of engine side coolant temperature ΔTe and the rate of change ofradiator side coolant temperature ΔTr is calculated from a map or amathematical expression by parameterizing at least one of the intake airamount GA, the engine speed Ne, the intake air temperature, the vehiclespeed and the operating state of the blower motor of the air-conditioner27 which are the parameters influencing the calorific value of theengine 11 and the radiation of the coolant in Step 2204 a.

Then, the rate ΔTe/ΔTr of the rate of change of engine side coolanttemperature ΔTe and the rate of change of radiator side coolanttemperature ΔTr is compared with the closure-malfunction discriminatingreference value ε in Step 2205 a. When the ΔTe/ΔTr≦ε, the processadvances to Step 2206 to determine that the thermostat 13 is normallyopened and to end the program.

When the ΔTe/ΔTr>ε on the other hand, the process advances to Step 2207to determine that the thermostat 13 has the closure-malfunction and endsthe program after lighting or flashing the alarm lamp 28 to warn thedriver of that in Step 2208 and storing the information on theclosure-malfunction in the backup RAM 29.

While the thermostat 13 has been diagnosed whether it has theopen-malfunction after the elapse of the predetermined time from thecold start in the second embodiment and its modification, theopen-malfunction may be diagnosed after an elapse of a predeterminedtime after when the thermostat 13 which has been opened is closed (afterT2 in FIGS. 25 and 26). That is, the open-malfunction may be diagnosedin the temperature range in which the thermostat 13 is normally closed.

Further, the intake pipe pressure may be used instead of the intake airamount and the outside-air temperature may be used instead of the intakeair temperature as the parameters used in calculating the malfunctiondiscriminating reference value.

Although the cooling fan 18 for cooling the radiator 15 is driven by thepower of the engine 11 in the embodiments having the system structureshown in FIG. 21, an electrically driven fan which is driven by anelectric motor may be used. Further, the position where the thermostat13 is mounted is not limited only to the outlet part of the water jacket12 and may be mounted at other parts such as the inlet part of the waterjacket 12.

Still more, because the behavior of the engine side coolant temperatureand the radiator side coolant temperature are influenced by themalfunction of the water pump 17, the radiator fan 18 and the blowermotor of the air-conditioner 27, it is possible to arrange so as todiagnose the malfunction of the water pump 17, the radiator fan 18 andthe blower motor of the air-conditioner 27 from the engine side coolanttemperature and the radiator side coolant temperature.

Further, the output signal of the radiator side coolant temperaturesensor 21 may be used as information for controlling the engine when thecoolant temperature sensor 20 is out of order. The second embodiment andits modifications may be arranged so as to implement only either one ofthe open-malfunction diagnosing program or the closure-malfunctiondiagnosing program.

Third Embodiment

The whole cooling system of the engine 11 of the this embodiment shownin FIG. 31 is the same as that of the first embodiment in which only onecoolant temperature sensor 20 is provided at the engine side. The ECU 22diagnoses the thermostat 13 whether it has the open-malfunction byexecuting each diagnosis routine shown in FIGS. 32 and 33 after enginewarm-up completion even under normal engine running condition whichfollows idling.

In FIG. 34, When the coolant temperature THW detected by the engine sidecoolant temperature sensor 20 exceeds the thermostat openingtemperature, the thermostat 13 opens when it is normal, so that the coldcoolant on the radiator 15 side flows into the engine 11 side tosuppress the coolant temperature from rising. Then, the coolanttemperature drops below the thermostat opening temperature. When thecoolant temperature THW drops below the thermostat closing temperatureafter that, the thermostat 13 is closed and the coolant is stopped fromcirculating from the radiator 15 side to the engine 11 side. Then, thecoolant on the engine 11 side is warmed up by the heat of the engine 11and the coolant temperature THW rises up more than the thermostatclosing temperature. Accordingly, the state in which the coolanttemperature THW drops largely below the thermostat closing temperaturedoes not continue for a long period of time.

Based on this point, the thermostat 13 is determined to have theopen-malfunction when the state in which the coolant temperature THWdrops below the malfunction discriminating temperature (e.g. 70° C.)which is lower than the thermostat closing temperature continues for apredetermined time since when the coolant temperature THW has exceededthe warm-up completion temperature (e.g. 80° C.) after the start of theengine in the this embodiment.

The thermostat open-malfunction diagnosing routine shown in FIG. 32 isinitiated per predetermined time (e.g. 32 ms). When this program isinitiated, data of the intake air temperature THA, the intake pipepressure PM and the coolant temperature THW output respectively from anintake air temperature sensor 25, an intake pipe pressure sensor 24 andthe coolant temperature sensor 20 in Step 3101. Then, it is determinedwhether the following malfunction diagnosing conditions (d) through (g)hold in Steps 3102 through 3105:

(d) A warm-up completion flag XTHW which is set in the routine in FIG.33 is “1” indicating that the warm-up has been completed. That is, thecoolant temperature THW has risen up more than 80° C. for example whichis the warm-up completion temperature (Step 3102);

(e) The intake air temperature THA is higher than 0° C. (Step 3103);

(f) The state in which the intake pipe pressure PM is larger than apredetermined value KPM (i.e. non-low load state) is continuing for morethan the predetermined time (Step 3104); and

(g) Fuel is supplied continuously, that is, fuel cut-off is notcontinuing for more than the predetermined time (Step 3105).

When all of these conditions (d) through (g) are satisfied (when thedeterminations of Steps 3102 through 3105 are all “Yes” ) indicatingthat the engine 11 is in other than the idling or deceleration, themalfunction diagnosing conditions hold. When there is even one conditionwhich is not satisfied, the malfunction diagnosing conditions do nothold and the this routine ends without implementing the diagnosis ofmalfunction.

Here, the conditions (e) through (g) (Steps 3103 through 3105) are whatfor determining whether it is the operating state during which thecoolant temperature THW is inclined to drop. When any one of Steps 3103through 3105 is “No”, i.e., the intake air temperature is THA≦0° C., thelow load state (PM<KPM) is continuing for more than the predeterminedtime, or the fuel cut-off is continuing for more than the predeterminedtime, it is the operating state during which the coolant temperature THWinclines to drop. When the operating state during which the coolanttemperature THW inclines to drop continues, the coolant temperature THWmay drop continuously and moderately even if the thermostat 13 isclosed, so that the discrimination of the malfunction is inhibited bythe processes in Steps 3103 through 3105 to prevent an erroneousdiscrimination of the open-malfunction of the thermostat 13 in advance.

When this malfunction diagnosing conditions hold, i.e. the warm-upcompletion flag XTHW=1 (warm-up is completed) and it is not theoperating state during which the coolant temperature THW inclines todrop (when all “Yes” in Steps 3102 through 3105), the process advancesto Step 3106 to determine whether or not the state in which the coolanttemperature THW drops below the malfunction discriminating temperature(e.g. 70° C.) which is lower than the thermostat closing temperature formore than the predetermined time To. When it is “Yes”, the processadvances to Step 3107 to determine that the thermostat 13 has theopen-malfunction and ends the routine after lighting or flashing thealarm lamp 28 to warn the driver of that and storing the malfunctioninformation in the backup RAM 29. When the state during which theTHW<70° C. (malfunction discriminating temperature reference value) isnot continuing for more than the predetermined time To on the otherhand, it is not determined to be the open-malfunction and the routineends.

It is noted that although it has been determined whether or not thethermostat 13 has the open-malfunction by determining whether the stateduring which the coolant temperature THW<70° C., the thermostat 13 maybe determined to have the open-malfunction when the coolant temperatureTHW drops below the malfunction discriminating temperature. Thereby, thethermostat 13 may be determined whether it has the open-malfunction ornot by setting the malfunction discriminating temperature at atemperature fully lower than the thermostat closing temperature.

The warm-up completion flag setting routine shown in FIG. 33 isinitiated per predetermined time (e.g. 32 ms) and reads the coolanttemperature THW detected by the coolant temperature sensor 20 at firstin Step 3111. Then, it is determined whether the warm-up completion flagXTHW is “o” indicating that the warm-up has not been completed in Step3112. When it has been set as XTHW=1 (warm-up is completed), the routineends as it is.

When XTHW=0 (warm-up is not completed), the process advances to Step3113 to determine whether the coolant temperature THW has exceeded 80°C., for example, which is the warm-up completion temperature. When ithas not exceeded 80° C., the routine ends as it is. When it has exceeded80° C., i.e. when the warm-up has been completed, the process advancesto Step 3114 to set the warm-up completion flag XTHW to “1” meaning thatthe warm-up has been completed and ends the routine. It is noted thatthe warm-up completion flag XTHW is reset to “0” by the initializationprocess at the starting time of the engine.

Because the open-malfunction of the thermostat 13 may be detected fromthe coolant temperature THW detected by the coolant temperature sensor20 in the third embodiment described above, no new sensor or the likefor detecting the open-malfunction is necessary, satisfying the demandof reducing the cost.

It is noted that it is possible to add a function of determining theclosure-malfunction of the thermostat 13 or the malfunction of theradiator fan 18 when the coolant temperature THW rises more than apredetermined temperature higher than the thermostat opening temperatureor when that state continues for the predetermined time. Further, theoutside air temperature may be used instead of the intake airtemperature in Step 3103.

Modification of Third Embodiment

A modification in which the third embodiment is applied to a vehicleprovided with an electronic throttle system will be explained belowbased on FIGS. 35 through 40. As described before, when the thermostat13 operates normally, the coolant temperature THW is controlled almostwithin the temperature range from the thermostat closing temperature tothe thermostat opening temperature (required coolant temperature range)and the state in which the coolant temperature THW is out of therequired coolant temperature range will not continue for a long time inthe normal operating state.

Based on this point, it is determined that the thermostat 13 has theclosure-malfunction in which it is kept closed when the coolanttemperature THW continuously rises up even when the predetermined timeTo has elapsed since when the coolant temperature THW has risen morethan the thermostat opening temperature as shown in FIG. 39 in thismodification. Further, it is determined that the thermostat 13 has theopen-malfunction in which it is kept opened when the coolant temperatureTHW continuously drops even when the predetermined time To has elapsedsince when the coolant temperature THW has dropped below the thermostatclosing temperature.

This modified diagnosis routine is shown in FIGS. 35 through 38. Thethermostat malfunction diagnosing routine shown in FIG. 35 is initiatedper predetermined time (e.g. 32 ms). When this program is initiated, thecoolant temperature THW detected by the coolant temperature sensor 20 isread in Step 3201. Then, it is determined in Step 3202 whether or not alow load flag XLOADL set/reset by the routine in FIG. 36 is “0”, i.e. amiddle load or high load range.

When XLOADL=0 (the middle load or high load range), it is determined inSteps 3203 through 3206 whether the thermostat 13 has theopen-malfunction or not. At first, it is determined whether or not thecoolant temperature THW is lower than the thermostat closing temperatureKTHWCL in Step 3203. When it is “Yes”, the process advances to Step 3204to determine whether or not the coolant temperature THW continuouslydrops. When it is “Yes”, it is determined whether or not thepredetermined time To during which the coolant temperature THWcontinuously drops has elapsed.

When the predetermined time To during which the coolant temperature THWcontinuously drops has elapsed, the process advances to Step 3206 todetermine that the thermostat 13 has the open-malfunction. Then, theroutine ends after lighting or flashing the alarm lamp 28 to warn thedriver of that and storing the malfunction information in the backup RAM29.

When it is determined to be “No” either in Step 3204 or 3205, i.e. thetime during which the coolant temperature THW continuously drops has notreached the predetermined time, on the other hand, it is not determinedto be the open-malfunction and the routine ends.

When it is determined to be “No” either in Step 3202 or 3203, i.e.XLOADL=1 (low load range) or the coolant temperature THW is more thanthe thermostat closing temperature KTHWCL, the process advances to Step3207 to determine whether or not a high load flag XLOADH set/reset inthe routine in FIG. 37 is “0”, i.e., the low load or middle load range.

When XLOADH=0 (the low load or middle load range), it is determined inSteps 3208 through 3211 whether the thermostat 13 has theclosure-malfunction or not. At first, it is determined whether or notthe coolant temperature THW is higher than the thermostat openingtemperature KTHWOP in Step 3208. When it is “Yes”, the process advancesto Step 3209 to determine whether or not the coolant temperature THWcontinuously rises. When it is “Yes”, it is determined whether or notthe predetermined time during which the coolant temperature THWcontinuously rises has elapsed.

When the predetermined time To during which the coolant temperature THWcontinuously rises has elapsed, the process advances to Step 3211 todetermine that the thermostat 13 has the closure-malfunction. Then, theroutine ends after lighting or flashing the alarm lamp 28 to warn thedriver of that and storing the malfunction information in the backup RAM29 and by limiting the throttle opening by a target throttle openingcomputing routine shown in FIG. 38 to limit the intake air amount tolimit the output of the engine (calorific heat value of the engine) andto limit ON of the air-conditioner 27 (drive of the compressor) to limitthe engine load in Step 3212.

Thus, switching to the control mode by which the intake air amount islimited and ON time of the air-conditioner is limited reduces the loadof the engine, prevent the engine from overheating and enables rimp-homerunning to a service station. It is noted that it is possible to arrangeso as to implement only either one of the limit of the intake air amountand the limit of ON of the air-conditioner.

When it is determined to be “No” in either one of Steps 3208 through3210, i.e., the coolant temperature THW is below the thermostat openingtemperature KTHWOP, or the time during which the coolant temperature THWcontinuously rises up does not reach the predetermined time To, it isnot determined to be the closure-malfunction and the routine ends.

The process in Step 3202 thus inhibits the discrimination of theopen-malfunction during the low load range and the process in Step 3207inhibits the discrimination of the closure-malfunction during the highload range. That is, because there is a case when the coolanttemperature THW continuously and moderately drops even when thethermostat 13 is closed in the low load range, an erroneousdiscrimination of the open-malfunction may be prevented by inhibitingthe discrimination of the open-malfunction. Further, because there is acase when the coolant temperature THW continuously and moderately riseseven when the thermostat 13 is opened in the high load range, anerroneous discrimination of the closure-malfunction may be prevented byinhibiting the discrimination of the closure-malfunction.

It is noted that although both the open-malfunction and theclosure-malfunction have been detected in the thermostat malfunctiondiagnosing routine in FIG. 35, it is possible to arrange so as to detectonly either one of the closure-malfunction and the open-malfunction.

The low load discriminating routine for Step 3202 in FIG. 35 is shown inFIG. 36 and is initiated per predetermined time (e.g., per 8 ms). Whenthis routine is initiated, the intake pipe pressure PM and the throttleopening TA are read in Step 3221 at first. Then, it is determinedwhether or not the present operating range is the low load range asfollows in Steps 3222 through 3225. Here, the low load range is theoperating range in which the coolant temperature THW may continuouslydrops even when the coolant temperature THW drops below the thermostatclosing temperature KTHWCL and the thermostat 13 is closed when thethermostat 13 operates normally.

At first, the intake pipe pressure PM is compared with a low loaddiscriminating value KPMLOADL in Step 3222. When the intake pipepressure PM is less than the low load discriminating value KPMLOADL, itis determined to be the low load range and the process advances to Step3227 to set the low load flag XLOADL to “1”. The low load discriminatingvalue KPMLOADL is set in correspondence with an engine speed at thatmoment by using a table or a mathematical expression of the KPMLOADL setby parameterizing the engine speed in advance.

When it is determined that the intake-pipe pressure PM is greater thanthe low load discriminating value KPMLOADL, the process advances to Step3223 to compare the throttle opening TA with the low load discriminatingvalue KPMLOADL. When the throttle opening TA is less than the low loaddiscriminating value KPMLOADL, it is determined to be the low load rangeand the process advances to Step 3227 to set the low load flag XLOADL to“1”. The low load discriminating value KPMLOADL is also set incorrespondence with the engine speed at that moment by using the tableor the mathematical expression in advance.

When it is determined in Step 3223 that the throttle opening TA isgreater than the low load discriminating value KPMLOADL, the processadvances to Step 3224 to determine whether fuel is supplied normally orcut off for deceleration. When the fuel is being cut off, it isdetermined to be the low load range and the process advances to Step3227 to set the low load flag XLOADL to “1”.

When the fuel is supplied normally without fuel cut-off, the processadvances to Step 3225 to determine whether the predetermined time duringwhich the three conditions of PM>PLMLOADL at Step 3222, TA>KTALOADL atStep 3223 and normal fuel supply at Step 3224 are all satisfied haselapsed. When the predetermined time has not elapsed, it is determinedto be the low load range and the process advances to Step 3227 to setthe low load flag XLOADL to “1” and to end the routine.

When it is determined to be “Yes” in all of Steps 3222 through 3225 onthe other hand, i.e. the predetermined time during which all theconditions are satisfied has elapsed, it is determined to be the middleload or the high load range. Then, the process advances to Step 3226 toset the low load flag XLOADL to “0” and to end the routine.

The high load discriminating routine for Step 3207 shown in FIG. 35 isshown in FIG. 37 and is initiated per predetermined time (e.g., per 8ms). When this routine is initiated, the intake pipe pressure PM, thethrottle opening TA and an air-conditioner signal AC are read in Step3231 at first. Then, it is determined whether or not the presentoperating range is the high load range as follows in Steps 3232 through3235. Here, the high load range is the operating range in which thecoolant temperature THW may continuously rise even when the coolanttemperature THW rises above the thermostat opening temperature KTHWOPand the thermostat 13 is opened when the thermostat 13 operatesnormally.

At first, the intake pipe pressure PM is compared with the high loaddiscriminating value KPMLOADH in Step 3232. When the intake pipepressure PM is more than the high load discriminating value KPMLOADH, itis determined to be the high load range and the process advances to Step3237 to set the high load flag XLOADH to “1”. The high loaddiscriminating value KPMLOADL is set in correspondence to an enginespeed at that moment by using a table or a mathematical expression ofthe KPMLOADH set by parameterizing the engine speed in advance.

When it is determined that the intake-pipe pressure PM is smaller thanthe high load discriminating value KPMLOADH in Step 3232, the processadvances to Step 3233 to compare the throttle opening TA with the highload discriminating value KPMLOADH. When the throttle opening TA isgreater than the high load discriminating value KPMLOADH, it isdetermined to be the high load range and the process advances to Step3237 to set the high load flag XLOADH to “1”. The high loaddiscriminating value KPMLOADH described above is also set incorrespondence with the engine speed at that moment by using the tableor the mathematical expression in advance.

When it is determined in Step 3233 that the throttle opening TA issmaller than the high load discriminating value KPMLOADH, the processadvances to Step 3234 to determine whether or not the air-conditionersignal AC is OFF. When it is not OFF (i.e., ON), it is determined to bethe high load range and the process advances to Step 3237 to set thehigh load flag XLOADH to “1”.

When the air-conditioner signal AC is OFF, the process advances to Step3235 to determine whether the predetermined time during which the threeconditions of PM<PLMLOADH at Step 3232, TA<KTALOADL at Step 3233 and theair-conditioner signal AC is OFF at Step 3234 are all satisfied haselapsed. When the predetermined time has not elapsed yet, it isdetermined to be the high load range and the process advances to Step3237 to set the high load flag XLOADH to “1” and to end the routine.

When it is determined to be “Yes” in all of Steps 3232 through 3235 onthe other hand, i.e., the predetermined time during which all thoseconditions are satisfied has elapsed, it is determined to be the lowload or the middle load range. Then, the process advances to Step 3236to reset the high load flag XLOADH to “0” and to end the routine.

The target throttle opening computing routine shown in FIG. 38 isinitiated per predetermined time (e.g., per 2 ms) and computes a targetthrottle opening TAC in correspondence with the control of anaccelerator pedal (not shown). The detail of the method for computingthe target throttle opening TAC is known in the art. After computing thetarget throttle opening TAC, it is determined in Step 3242 whether thethermostat 13 has the open-malfunction or not based on the result of thediagnosis of the malfunction by the routine shown in FIG. 35. When it isnot the open-malfunction, the routine ends as it is. In this case, theaperture of the throttle valve is controlled in response to the targetthrottle opening TAC computed in Step 3241.

When the open-malfunction has occurred on the other hand, the processadvances from Step 3242 to Step 3243 to correct the target throttleopening TAC by multiplying a correction factor a to the target throttleopening TAC calculated in Step 3241. Here, the target throttle openingTAC at the time of open-malfunction is reduced as compared with thatduring the normal time by setting the correction factor α<0 to limit theintake air amount. It is noted that a map for calculating the targetthrottle opening TAC during the open-malfunction may be prepared andstored in the ROM in advance to calculate the target throttle openingTAC during the open-malfunction from this map.

Then, a lower limit guarding process is implemented so that the targetthrottle opening TAC during the open-malfunction will not become toosmall in Step 3224 and then the routine ends.

It is noted that it is possible to arrange so as to limit the intake airamount and to limit ON time of the air-conditioner (switching to thecontrol mode during a malfunction) when a malfunction other than that ofthe thermostat 13 (such as a malfunction of the radiator fan 18, thedrop of coolant level and the like) occurs in the engine cooling system.

Further, each routine shown in FIGS. 35 through 37 may be applied to avehicle not provided with the electronic throttle. In this case, ON/OFFof the air-conditioner is implemented in Step 3212 in FIG. 35.

While preferred embodiments and modifications thereof have beendescribed, further variations thereto will occur to those skilled in theart within the scope of the present inventive concepts which aredelineated by the following claims.

What is claimed is:
 1. A thermostat malfunction detecting system for aninternal combustion engine thermostat provided in a coolant circulatingpath, said system comprising: coolant temperature detecting means inaddition to said thermostat for detecting coolant temperature on thecoolant circulating path on the engine side from said thermostat;temperature determining means which responds to said temperaturedetecting means for determining whether the coolant temperature detectedupon starting said engine is lower than an opening temperature of saidthermostat; and open-malfunction diagnosing means for diagnosing, whenthe coolant temperature detected upon starting said engine is lower thanthe opening temperature, whether said thermostat has an open-malfunctionbased on the engine side coolant temperature detected by said coolanttemperature detecting means in a temperature range in which saidthermostat is normally to be closed.
 2. The thermostat malfunctiondetecting system of claim 1, wherein said open-malfunction diagnosingmeans determines a decrease or a rate of drop of the engine side coolanttemperature right after the start of the engine and diagnoses whethersaid thermostat has the open-malfunction based on the decrease or therate of drop of the engine side coolant temperature.
 3. The thermostatmalfunction detecting system of claim 1, wherein: said open-malfunctiondiagnosing means determines an increase of the engine side coolanttemperature until when any one of (i) an elapsed time after the start ofthe engine, (ii) the number of times of ignition and (iii) anaccumulated value of quantity of heat generated by engine, reaches apredetermined value and diagnoses whether said thermostat has theopen-malfunction based on the increase of the engine side coolanttemperature.
 4. The thermostat malfunction detecting system of claim 1,wherein: said open-malfunction diagnosing means determines any one of(i) an elapsed time after the start of the engine, (ii) the number oftimes of ignition and (iii) an accumulated value of quantity of heatgenerated by engine, until when the increase of the engine side coolanttemperature reaches a predetermined value or until when the engine sidecoolant temperature reaches a predetermined temperature and diagnoseswhether said thermostat has the open-malfunction based on a referencevalue.
 5. The thermostat malfunction detecting system of claim 1,wherein: said open-malfunction diagnosing means determines the increaseof the engine side coolant temperature after the start of the engine anddiagnoses whether said thermostat has the open-malfunction based on anumber of times when the increase of the engine side coolant temperaturebecomes less than a predetermined value.
 6. The thermostat malfunctiondetecting system of claim 5, wherein said open-malfunction diagnosingmeans determines the increase of the engine side coolant temperature byeither one of (i) the increase of coolant temperature per predeterminedtime, (ii) the increase of coolant temperature per predetermined numberof times of ignition and (iii) the increase of coolant temperature perpredetermined quantity of heat generated by engine.
 7. The thermostatmalfunction detecting system of claim 1, wherein: said open-malfunctiondiagnosing means corrects data used for the diagnostic process of theopen-malfunction based on at least any one of (i) a vehicle speed, (ii)an outside air temperature, (iii) an intake air temperature and (iv) anoperating state of an air-conditioner.
 8. The thermostat malfunctiondetecting system of claim 2, wherein: said open-malfunction diagnosingmeans diagnoses the malfunction after an elapse of a predetermined timefrom a start of the engine.
 9. The thermostat malfunction detectingsystem of claim 2, wherein: said open-malfunction diagnosing meansdiagnoses the malfunction while the engine is in an idle state.
 10. Athermostat malfunction detecting system according to claim 7, whereinsaid open-malfunction diagnosing means: detects the malfunction when thecoolant temperature is kept lower than a reference temperature after anelapse of a predetermined time from starting of said engine, and variesthe reference temperature with an accumulated value of heat generated bysaid engine.
 11. A thermostat malfunction detecting system for aninternal combustion engine thermostat provided between an engine and aradiator on a coolant circulating path for cooling the engine, saidsystem comprising: engine side coolant temperature detecting means fordetecting a coolant temperature on the coolant circulating path on theengine side from said thermostat; radiator side coolant temperaturedetecting means for detecting a coolant temperature on the coolantcirculating path on the radiator side from said thermostat; andmalfunction diagnosing means for diagnosing whether said thermostat hasa malfunction based on the engine side coolant temperature and theradiator side coolant temperature detected by said engine side andradiator side coolant temperature detecting means.
 12. The thermostatmalfunction detecting system of claim 11, wherein: said malfunctiondiagnosing means has open-malfunction detecting means for diagnosingwhether said thermostat has an open-malfunction in which said thermostatis kept opened in the temperature range in which said thermostatnormally closes.
 13. The thermostat malfunction detecting system ofclaim 12 wherein: said open-malfunction diagnosing means diagnoseswhether said thermostat has the open-malfunction or not during theperiod from a cold start of the engine to a time when the coolantreaches the temperature at which said thermostat normally opens.
 14. Thethermostat malfunction detecting system of claim 12, wherein: saidopen-malfunction diagnosing means diagnoses whether said thermostat hasthe open-malfunction or not based on a difference of temperature betweenthe engine side coolant temperature and the radiator side coolanttemperature detected by said engine side and radiator side coolanttemperature detecting means.
 15. The thermostat malfunction detectingsystem of claim 12, wherein: said open-malfunction diagnosing meansdiagnoses whether said thermostat has the open-malfunction or not basedon a rate of change of the engine side coolant temperature detected bysaid engine side and radiator side coolant temperature detecting means.16. The thermostat malfunction detecting system of claim 11, wherein:said malfunction diagnosing means has discriminating reference valuesetting means for setting a malfunction discriminating reference valuebased on at least one of (i) operating condition of said engine, (ii)outside-air temperature, (iii) intake air temperature and (iv) anoperating condition of an air-conditioner.